Thursday, January 2, 2014

Pterosaur Aerodynamics at GWU

+Lorena Barba over at George Washington University had her students do some fun summaries of pterosaur aerodynamics at the end of the Fall Semester. She is a professor in engineering who runs a course on animal flight (she is also the CFD guru behind the latest analysis of gliding snakes with J. Socha). These topics followed from a discussion that Lorena and I had after some interesting (and amusing) trolling on my Google+ stream. We identified four topics for her students to look into more: pterosaur launch, pterosaur weight estimates, pterosaur wings and flight capabilities, and Wagner Effects. The students wrote blog posts, which can be found here:

These students are all engineers, which adds a fun element because most were quite unfamiliar with pterosaur biology going into this project. There are some errors in the terminology and details of biology as a result, but it's quite good work, and brought some new things to light for me (especially the Wagner section). Enjoy!

9 comments:

Dear Lorena Barba,I enjoyed the guest blogs done by your students. Although the first three were based on the well-known Habib/Witton/Palmer canon, I was more interested in the blog on the Wagner effect.

There were still a few howler however. In the Druv/Golding blog they state that "Tupuxuara weighs in at 22.8 kg, nearly half the weight of the largest flying modern birds at 41 kg."As far as I know, the heaviest modern flying bird is the Kori Bustard at 19Kg. I am not aware of any bird flying at 41Kg. Perhaps this is a typo for lbs rather than kg? If so, it should really have been checked before release.

Another one was the delightfully simplistic 4th part of the quadrepedal launch sequence: I quote: "Launch —In the final phase, the pterosaur completed its quadrupedal launch and took off." This is phraseology straight out of a childrens' novel and not really scientific language at all.

Finally, there was the maths that went awry on the blog on weight estimation (Forcha) where we appear to have mention of pterosaur wing loadings of 220Kg/m2. That is greater than a B-17 Bomber! I suspect this should have read 22Kg/m2. Another typo perhaps but they all detract from the credibility of the whole article.

I have managed to get under the skin of several palaeontologists including Habib, Witton, Palmer et al. From my reading of learned articles, it would appear that the palaeontological community have become used to making statements about what these extinct creatures could and couldn't do.

It is easy for palaeontologists to pontificate on the taxonomy of ground or even sea-dwelling creatures, with little fear of contradiction. Who can say how heavy a Tricerotops weighed or how fast a Plesiosaur could swim? However, when it comes to Pterosaurs, they are rigidly bound by the laws of aerodynamics. Habib has postulated that Quetzalcoatlus N. could fly at 80 mph; fly non-stop half way around the globe at 15,000 ft. And that ridiculous theory of the quad launch! His arguments are that some modern birds can do it so why can’t Quetzy. My counter argument is that Penguins, Hummingbirds and Gannets are also birds – does that mean that Quetzalcoatlus could swim, hover and dive? We need to get away from the theatrical postulations of recent years. Let’s try instead to work out whether and how these creatures lived and – if indeed they did - flew.

OK No feedback? I am an aerodynamicist. If we are going to get the big, late-Cretaceous pterosaurs to fly we HAVE to have all their dimensions in order to fit in with aerodynamic theory. We have lengths, widths etc from fossils, but no-one can come up with a weight that matches. We must rely on palaeontologists to come up with a professional estimate for us. With Quetzalcoatlus we have been given a 30ft wingspan with (according to Mark Witton) a body only 60cm long. Yes, 60 cm. The height of a giraffe but weighing only 250Kg. This is the creature that Habib has flying at 15,000 ft and travelling at 80mph half way around the world non-stop. Any of you students like to respond? Or are you content to follow the palaeontologists and just accept everything they throw at you - melodramatic, theatrical or downright contradictory to the laws of physics?Phil Parsons

Dear Lorena,I'm not getting any response from your students. Are they just copying chunks of other people's work and not really understanding it? Please don't let them follow Mike Habib into his wonderful world of the light fantastic. Get them to THINK through what they are writing and stand by it rather than rolling over and doing the sycophant in the face of these 'famous' palaeontologists. Remind them of the story of the Emperors New Clothes. Emperor Habib can give them plenty of opportunity!Phil Parsons

Dear Phil Parsons,I am one of Prof. Barba's graduate student, and have been working on the Wagner effect post. In you first comment, you said we were more interested in this post, but reading what your next comments, I see nothing related to that. I will be happy to discuss about it with you if you let me know your thoughts.Best,Olivier Mesnard

Hi OlivierThank you for responding to my posts. My problem in a nutshell is that I am annoyed with Messrs Habib, Witton, Palmer et al for some of the claims they are making concerning the flight capabilities of the larger, Azhdarchid pterosaurs.Their claims are simply not sustainable using the laws of physics!

Why? Well firstly let me surmise that the bulk of palaeontologists focus on land and sea dwelling creatures. They must surely be used to making sweeping statements about the size and weight of Tyrannosaurs or Plesiosaurs without anyone being able to contradict them. Land animals walk and sea creatures swim and that's about it. So sweeping statements on capabilities don't lend themselves to criticism.

However, if a creature is to fly it faces a considerable hurdle in the form of the laws of aerodynamics. Habib and Co have extrapolated a single arm bone from Quetzalcoatlus Northropii into an almost mystical creature capable of 80mph flight at 15,000ft and cruising 12,000 miles in 8 days non-stop etc etc.

I mentioned earlier the story of the Emperor's Clothes? Well, Habib and Co make fine Emperors who have clothed themselves in proposals and theories which, if you take a pause and look closely at them, are plainly not scientific and in my view are just risible. Habib has even admitted that his calculations cannot be published 'because he is still working on them'. Talk about a snake-oil salesman. (And you can tell him I said so!)

Let's look at your paper on the Wagner effect. I put it to you that the Wagner effect can only apply to small, narrow chord wings. It could be argued that the small, bird-sized pterosaurs might be affected by vortex flow, but Q.n apparently had 30ft wings with Re of the order of 10^6 so I think we would be safe keeping to laminar flow formulae, even for flapping fight. Certainly there will be vortices, but not lift-inducing ones - just drag don't you think?.

As you know, at low airspeed and narrow wing chord, air viscosity is a dominant factor. The bigger the chord and higher the speed, Reynolds numbers become so high that air viscosity becomes insignificant. With Q.n we are not dealing with a bumblebee, dragonfly or even a sparrow. We are dealing with a 12m span (lets say 4m per flapping wing) and with an aspect ratio which appears to give a mean chord of about 1.5m. Re = (air density/viscosity) x airspeed x chord. If the Cretaceous atmosphere was the same as today's, for a 250Kg creature, the minimum velocity for a wing area of about 13m2 would have to be about 20ms so the formula would result in Re=68459x1.5x20. Reynolds would be is 205,3770 or 10^6 plus. Looks like the Wagner effect is a non-starter wouldn't you say?

Are you interested in more scepticism from me? I'd be delighted if you would join the 'sensible' side of the pterosaur arguments.

Greetings Mr. Parsons. As always, I appreciate your interest in the subject, but I suspect the reason you aren't getting responses from students or other blog posters here is because your attitude is problematic. Disagreement is fine. Making ad hominem attacks and the like, on the other hand, are not. It's also not helpful to spam comment streams with one liners like "Come on Lorena Barba's team. What about some feedback?"

I don’t particularly care that you disagree with me, Colin, or anyone else. One cannot be a professional scientist while being fearful of disagreement, after all. However, I do care that you practice some basic etiquette on the blog here at pterosaur.net. I would hope it is obvious that personal attacks, for example, do not constitute a reasonable approach to scientific discussion.

Another reason that you may not be getting many responses is that you self-describe as an aerodynamicist and then make rather critical errors that undermine this label. For example, at high Re numbers flows become less laminar, not more laminar. Similarly, vorticity is required for lift. Any lifting structures is producing a bound vortex, otherwise it wouldn’t be producing lift. Low Re flows will produce additional vorticity that is mostly absent at higher Re ranges, such as leading edge vortices, but your comment that “Certainly there will be vortices, but not lift-inducing ones - just drag don't you think?” suggests a fundamental misunderstanding regarding vorticity. Those are pretty basic mistakes.

Keep in mind that Colin Palmer, Jim Cunningham, and I are aerodynamicists. So your argument from authority approach rings rather hollow. It is also telling that the results you find so outrageous have not raised eyebrows among lead aerodynamicists at Lockheed Martin or Scaled Composites (just to use two examples).

I think you will find it difficult to stimulate discussion if you argue that we can’t even measure fossils. That 60 cm estimate you are up in arms over is based on direct measurements. Quetzalcoatlus is known from nearly complete skeletons. The largest specimen (Q. northopi) is known from a wing (not a single bone). Assuming that the proportions of the large animal were similar to the small ones, the shoulder to hip distance would have been about 0.60 meters. Among azhdarchids, the skull length tend to be about 3-4.5 times the shoulder to hip distance. Are those unusual proportions compared to living animals? Yes. But those are the real proportions. You can be incredulous all you want, but it does not change the measurements.

Your unsupported incredulity about unremarkable numbers doesn’t help. (15,000 feet is not an extreme altitude for a long-range flyer with a flow-through lung system, for example, and even a passing look at the literature shows that quite a few birds hit that altitude during migration). The same goes for your disdain for my launch reconstructions. If you have serious evidence for bipedal launch, then please do share (or better yet, publish it). However, acting as if it is somehow obviously absurd just makes you seem unfamiliar with the basics of animal launch.

Baiting is also not a good approach to attracting discussion. My previous quantitative work is published, including information on methodology. Ongoing work is not yet published - that’s why it’s called ongoing. Some of that work has been presented at conferences. I refused to share all of the ongoing calculations and measurements in a public blog prior to peer reviewed publication. That shouldn’t be a surprise.

I’ll be honest and say that I can’t quite figure out why this work angers you so much. Maybe some of my estimates or predictions will turn out to be wrong someday. That is simply part of science. If you have work that suggests different conclusions then by all means publish it! That’s how we build better models. But I really don’t understand why you are so annoyed that some random professor you’ve never met thinks some extinct reptiles flew well.

Greetings Dr Habib,Thank you for at last acknowledging that I am here and arguing my position.I am not really interested in battling you with scientific or even aerodynamic formulae. That does not get us anywhere. The reason for my scepticism is that you have been making blatant assumptions about the flight of the large Azhdarchids and then extrapolating these claims beyond believeable reason.I believe firmly that these creatures could fly - what I cannot understand is HOW. Your extravagant claims concerning their flight capablities just annoys the heck out of me because they are not supported by reason let alone science.I have assembled a Witton/Habib flying machine with its 60cm body and its 11m span wing and put it together in a photograph. It is actually a large dog and a hang glider wing which apparently fits your specification for Q.n.

I will try and put it up here if I can work out how to do it. I'm not sure I can upload photographs on this site. Watch this space.....